Heat olive oil in a skillet over medium-high heat. Add chicken, onion, and bell peppers to the hot skillet, sprinkle with chili powder and salt, and stir to coat. Sauté until chicken is cooked through and peppers are tender, about 10 minutes.
While chicken cooks, ppray a 9 x 13″ pan with cooking spray, and add 2/3 c. enchilada sauce. Swirl to coat the bottom of the pan. Arrange six tortillas over the sauce. (It’s OK if they overlap a bit.)
Drop dollops of the refried beans over the tortillas, and spread them into an even layer over the tortillas. You may need to hold the tortillas still with your finger as you spread the beans to keep them from sliding out of place. Sprinkle half the cheese over the beans.
When chicken is done, evenly scoop hot chicken and vegetables over the cheese layer, leaving the liquid behind in the pan.
Drizzle 1/3 c. enchilada sauce over the chicken. Arrange remaining tortillas evenly over chicken (again, overlapping is OK), and pour about a cup of enchilada sauce over the tortillas. Use your fingers or a spatula to spread the sauce to fully coat the tortillas.
Sprinkle the remaining cheese over the top, and bake uncovered in the preheated oven for 30 minutes, or until cheese is melted and sauce is bubbly. Allow to stand a few minutes before cutting and serving. Top with lettuce and sour cream if desired.